On the first 4 tests of his math class, Ben got an average score of 84. What does he need on the next test to have an overall average of 87?
Answer: Let his score on the next test be $x$ The sum of all of his scores is then $4 \cdot 84 + x$ The same sum must also be equal to $5 \cdot 87$ Solve: $x = 5 \cdot 87 - 4 \cdot 84 = 99$.